A step-ratio automatic transmission system in a vehicle utilizes multiple friction elements for automatic gear ratio shifting. Broadly speaking, these friction elements may be described as torque establishing elements although more commonly they are referred to as clutches or brakes. The friction elements function to establish power flow paths from an internal combustion engine to vehicle traction wheels. During acceleration of the vehicle, the overall speed ratio, which is the ratio of a transmission input shaft speed to a transmission output shaft speed, is reduced during a ratio upshift as vehicle speed increases for a given engine throttle setting. A downshift to achieve a higher speed ratio occurs as an engine throttle setting increases for any given vehicle speed, or when the vehicle speed decreases as the engine throttle setting is decreased.
Various planetary gear configurations are found in modern automatic transmissions. However, the basic principle of shift kinematics remains similar. Shifting a step-ratio automatic transmission having multiple planetary gearsets is accompanied by applying and/or releasing friction elements to change speed and torque relationships by altering the torque path through the planetary gearsets. Friction elements are usually actuated either hydraulically or mechanically.
In the case of a synchronous friction element-to-friction element upshift, a first pressure actuated torque establishing demerit, referred to as an off-going friction element, is released while a second pressure actuated torque establishing element, referred to as an on-coming friction element, engages in order to lower a transmission gear ratio. A typical upshift event is divided into preparatory, torque and inertia phases. During the preparatory phase, an on-coming friction element piston is stroked to prepare for its engagement while an off-going friction element torque-holding capacity is reduced as a step toward its release. During the torque phase, which may be referred to as a torque transfer phase, on-coming friction element torque is raised while the off-going friction element is still engaged. The output shaft torque of the automatic transmission typically drops during the torque phase, creating a so-called, torque hole. When the on-coming friction element develops enough torque, the off-going friction element is released, marking the end of the torque phase and the beginning of the inertia phase. During the inertia phase, the on-coming friction element torque is adjusted to reduce its slip speed toward zero. When the on-coming friction element slip speed reaches zero, the shift event is completed.
In a synchronous shift, the timing of the off-going friction element release must be synchronized with the on-coming friction element torque level to deliver a consistent shift feel. A premature release leads to engine speed flare and a deeper torque hole, causing perceptible shift shock for a vehicle occupant. A delayed release causes a tie-up of gear elements, also resulting in a deep and wide torque hole for inconsistent shift feel. A conventional shift control relies on speed measurements of the powertrain components, such as an engine and a transmission input shaft, to control the off-going friction element release process during the torque phase. A conventional torque phase control method releases the off-going friction element from its locked state through an open-loop control based on a pre-calibrated timing, following a pre-determined off-going friction element actuator force profile. This conventional method does not ensure optimal off-going friction element release timing and therefore results in inconsistent shift feel.
Alternatively, a controller may utilize speed signals to gauge off-going friction element release timing. That is, the off-going friction element is released if the controller detects a sign of gear tie-up, which may be manifested as a measurable drop in input shaft speed. When a release of the off-going friction element is initiated prematurely before the on-coming friction element develops enough torque, engine speed or automatic transmission input shaft speed may rise rapidly in an uncontrolled manner. If this so-called engine speed flare is detected, the controller may increase off-going friction element control force to quickly bring down automatic transmission input speed or off-going friction element slip speed. This speed-based or slip-based approach often results in a hunting behavior between gear tie-up and engine flare, leading to inconsistent shift feel. Furthermore, off-going friction element slip control is extremely difficult because of its high sensitivity to slip conditions and a discontinuity between static and dynamic frictional forces. A failure to achieve a seamless slip control during the torque phase leads to undesirable shift shock.
In the case of a non-synchronous automatic transmission, the upshifting event involves engagement control, of only an on-coming friction element, while a companion clutching component, typically a one-way coupling, automatically disengages to reduce the speed ratio. The non-synchronous upshift event can also be divided into three phases, which may also be referred to as a preparatory phase, a torque phase and an inertia phase. The preparatory phase for the non-synchronous upshift is a time period prior to the torque phase. The torque phase for the non-synchronous shift is a time period when the on-coming friction element torque is purposely raised for its engagement until the one-way coupling starts slipping or overrunning. This definition differs from that for the synchronous shift because the non-synchronous shift does not involve active control of a one-way coupling or the off-going friction element. The inertia phase for the non-synchronous upshift is a time period when the one-way coupling starts to slip, following the torque phase. According to a conventional upshift control, during the torque phase of the upshifting event for a non-synchronous automatic transmission, the torque transmitted through the oncoming friction element increases as it begins to engage. A kinematic structure of a non-synchronous upshift automatic transmission is designed in such a way that torque transmitted through the one-way coupling automatically decreases in response to increasing oncoming friction element torque. As a result of this interaction, the automatic transmission output shaft torque drops during the torque phase, which again creates a so-called “torque hole.” Before the one-way coupling disengages, as in the case previously described, a large torque hole can be perceived by a vehicle occupant as an unpleasant shift shock. An example of a prior art shift control arrangement can be found in U.S. Pat. No. 7,351,183, which is hereby incorporated by reference.
A transmission schematically illustrated at 2 in FIG. 1 is an example of a prior art multiple-ratio transmission with a controller 4 wherein ratio changes are controlled by friction elements acting on individual gear elements. Engine torque from an input torque source such as vehicle engine 5 is distributed to torque input element 10 of hydrokinetic torque converter 12. An impeller 14 of torque converter 12 develops turbine torque on a turbine 16 in a known fashion. Turbine torque is distributed to a turbine shaft, which is also transmission input shaft 18. Transmission 2 of FIG. 1 includes gearing in the form of a simple planetary gearset 20 and a compound planetary gearset 21. Gearset 20 has a permanently fixed sun gear S1, a ring gear R1 and planetary pinions P1 rotatably supported on a carrier 22. Transmission input shaft 18 is drivably connected to ring gear R1. Compound planetary gearset 21, sometimes referred to as a Ravagineaux gearset, has a small pitch diameter sun gear S3, a torque output ring gear R3, a large pitch diameter sun gear S2 and compound planetary pinions. The compound planetary pinions include long pinions P2/3, which drivably engage short planetary pinions P3 and torque output ring gear R3. Long planetary pinions P2/3 also drivably engage short planetary pinions P3. Short planetary pinions P3 further engage sun gear S3. Planetary pinions P 2/3, P3 of gearset 21 are rotatably supported on compound carrier 23. Ring gear R3 is drivably connected to a torque output member or shaft 24, which is drivably connected to vehicle traction wheels through a differential and axle assembly (not shown). Gearset 20 is an underdrive ratio gearset arranged in series with respect to compound gearset 21. Typically, transmission 2 preferably includes a lockup or torque converter bypass clutch, as shown at 25, to directly connect transmission input shaft 18 to engine 5 after a torque converter torque multiplication mode is completed and a hydrokinetic coupling mode begins. FIG. 2 is a chart showing a clutch and brake friction element engagement and release pattern for establishing each of six forward driving ratios and a single reverse ratio for transmission 2.
During operation in the first four forward driving ratios, carrier P1 is drivably connected to sun gear S3 through shaft 26 and forward friction element A. During operation in the third ratio, fifth ratio and reverse, direct friction element B drivably connects carrier 22 to shaft 27, which is connected to large pitch diameter sun gear S2. During operation in the fourth, fifth and sixth forward driving ratios, overdrive friction element E connects turbine shaft 18 to compound carrier 23 through shaft 28. Friction element C acts as a reaction brake for sun gear S2 during operation in second and sixth forward driving ratios. During operation of the third forward driving ratio, direct friction element B is applied together with forward friction element A. The elements of gearset 21 then are locked together to effect a direct driving connection between shaft 28 and output shaft 26. The torque output side of forward friction element A is connected through torque transfer element 29 to the torque input side of direct friction element B, during forward drive. The torque output side of direct friction element B, during forward drive, is connected to shaft 27 through torque transfer element 30. Reverse drive is established by applying low-and-reverse brake D and friction element B.
For the purpose of illustrating one example of a synchronous ratio upshift for the transmission of FIG. 1, it will be assumed that an upshift will occur between the first ratio and the second ratio. On such a 1-2 upshift, a first pressure actuated torque establishing element, friction element C, starts in the released position before the shift and is engaged during the shift while a second pressure actuated torque establishing element, low/reverse friction element D, starts in the engaged position before the shift and is released during the shift. Forward friction element A stays engaged while friction element B and overdrive friction element F stay disengaged throughout the shift. More details of this type of transmission arrangement are found in U.S. Pat. No. 7,216,025, which is hereby incorporated by reference.
FIG. 3 depicts a general process of a synchronous friction element-to-friction element upshift event from a low gear con figuration to a high gear configuration for the automatic transmission system of FIG. 1. For example, the process has been described in relation to a 1-2 synchronous ratio upshift above wherein friction element C is an oncoming friction element and low/reverse friction element D is an off-going friction element, but it is not intended to illustrate a specific control scheme.
The shift event is divided into three phases: a preparatory phase 31, a torque phase 32 and an inertia phase 33. During preparatory phase 31, an on-coming friction element piston is stroked (not shown) to prepare for its engagement. At the same time, off-going friction element control force is reduced as shown at 34 as a step toward its release. In this example, off-going friction element D still retains enough torque capacity shown at 35 to keep it from slipping, maintaining transmission 2 in the low gear configuration. However, increasing on-corning friction element control force shown at 36 reduces net torque flow within gearset 21. Thus, the output shaft torque drops significantly during torque phase 32, creating a so-called torque hole 37. A large torque hole can be perceived by a vehicle occupant as an unpleasant shift shock. Toward the end of torque phase 32, off-going friction element control force is dropped to zero as shown at 38 while on-coming friction element apply force continues to rise as shown at 39. Torque phase 32 ends and inertia phase 33 begins when off-going friction element D starts slipping as shown at 40. During inertia phase 33, off-going friction element slip speed rises as shown at 41 while on-corning friction element slip speed decreases as shown at 42 toward zero at 43. The engine speed and transmission input speed 44 drops as the planetary gear configuration changes. During inertia phase 33, output shaft torque indicated by profile 45 is primarily affected by on-coming friction element C torque capacity indirectly indicated by force profile 46. When on-coming friction element C completes engagement or when its slip speed becomes zero at 43, inertia phase 33 ends, completing the shift event.
FIG. 4 shows a general process of a synchronous friction element-to-friction element upshift event from the low gear configuration to the high gear configuration in which off-going friction element D is released prematurely as shown at 51 compared with the case shown in FIG. 3. When off-going friction element D is released, it breaks a path between automatic transmission input shaft 18 and automatic transmission output shaft 24, depicted in FIG. 1, no longer transmitting torque to automatic transmission output shaft at the low gear ratio. Since on-coming friction element C is yet to carry enough engagement torque as indicated by a low apply force at 52, automatic transmission output shaft torque drops significantly, creating a deep torque hole 53 which can be felt as a shift shock. At the same time, engine speed or transmission input speed rapidly increases as shown at 54, causing a condition commonly referred to as engine flare. A large level of engine flare can be audible to a vehicle occupant as unpleasant noise. Once on-coming friction element C develops sufficient engagement torque as indicated by a rising control force at 55, automatic transmission input speed comes down and the output torque rapidly moves to a level at 56 that corresponds to on-coming friction element control force 55. Under certain conditions, this may lead to a torque oscillation 57 that can be perceptible to a vehicle occupant as unpleasant shift shock.
FIG. 5 shows a general process of a friction element-to-friction element upshift event from the low gear configuration to the high gear configuration in which off-going friction element release is delayed as shown at 61 compared with the case shown in FIG. 3. Off-going friction element D remains engaged even after on-coming friction element C develops a large amount of torque as indicated, by a large actual control force at 65. Thus, transmission input torque continues to be primarily transmitted to output shaft 24 at the low gear ratio. However, large on-coming friction element control force 65 results in a drag torque, lowering automatic transmission output shaft torque, creating a deep and wide torque hole 63. This condition is commonly referred to as a tie-up of gear elements. A severe tie-up can be felt as a shift shock or loss of power by a vehicle occupant.
As illustrated in FIGS. 3, 4, and 5 a missed synchronization of off-going friction element release timing with respect to on-coming friction element torque capacity leads to engine flare or tie-up. Both conditions lead to varying torque levels and profiles at automatic transmission torque output shaft 24 during shifting. If these conditions are severe, they result in undesirable driving experience such as inconsistent shift feel or perceptible shift shock. The prior art methodology attempts to mitigate the level of missed-synchronization by use of an open loop off-going friction element release control based on speed signal measurements. It also attempts to achieve a consistent on-coming friction element engagement torque by an open-loop approach during a torque phase under dynamically-changing shift conditions.
FIG. 6 illustrates a prior art methodology for controlling a friction element-to-friction element upshift from a low gear configuration to a high gear configuration for automatic transmission 2 in FIG. 1. The prior art on-coming control depicted in FIG. 6 applies to a conventional torque phase control utilized for either a synchronous or non-synchronous shift. In this example, off-going friction element D remains engaged until the end of torque phase 32. Although the focus is placed on torque phase control, FIG. 6 depicts the entire shift control process. As shown, the shift event is divided into three phases: a preparatory phase 31, a torque phase 32 and an inertia phase 33. During preparatory phase 31, an on-coming friction element piston is stroked (not shown) to prepare for its engagement. At the same time, off-going friction element control force is reduced as shown at 34 as a step toward its release. During torque phase 32, controller 4 commands an on-coming friction element actuator to follow a prescribed on-coming friction element control force profile 64 through an open-loop based approach. Actual on-coming friction element control force 65 may differ from prescribed profile 64 due to control system variability. Even if actual control force 65 closely follows prescribed profile 64, on-coming friction element engagement torque may still vary significantly from shift to shift due to the sensitivity of the on-coming friction element engagement process to engagement conditions such as lubrication oil flow and friction surface temperature. Controller 4 commands enough off-going element control force 61 to keep off-going element D from slipping, maintaining the planetary gearset in the low gear configuration until the end of torque phase 32. Increasing on-coming friction element control force 65 or engagement torque reduces net torque flow within the low-gear configuration. Thus, output shaft torque 66 drops significantly during torque phase 32, creating so-called torque hole 63. If the variability in on-coming friction element engagement torque significantly alters a shape and depth of torque hole 63, a vehicle occupant may experience inconsistent shift feel. Controller 4 reduces off-going friction element actuator force at 38, following a pre-calibrated profile, in order to release it at a pre-determined timing 67. The release timing may be based on a commanded value of on-coming friction element control force 62. Alternatively, off-going friction element D is released if controller 4 detects a sign of significant gear tie-up, which may be manifested as a detectable drop in input shaft speed 44. Inertia phase 33 begins when off-going friction, element D is released and starts slipping as shown at 67. During inertia phase 33, off-going friction element slip speed rises as shown at 68 while on-coming friction element slip speed decreases toward zero as shown at 69. Transmission input speed 44 drops as the planetary gear configuration changes. During inertia phase 33, output shaft torque 66 is primarily affected by on-corning friction element torque capacity or control force 65. The shift event completes when the on-coming friction element comes into a locked or engaged position with no slip as shown at 70.
FIG. 7 illustrates another prior art methodology for controlling torque phase 32 of a synchronous upshift process from the low gear configuration to the high gear configuration. In this example, controller 4 allows off-going friction element D to slip during torque phase 32. Although the focus is placed on torque phase control, FIG. 7 depicts the entire shift event. During preparatory phase 31, an on-coming friction element piston is stroked to prepare for its engagement. At the same time, off-going friction element control force 86 is reduced as a step toward its slip. During torque phase 32, on-coming friction element control force is raised in a controlled manner. More specifically, controller 4 commands on-coming friction element actuator to follow a prescribed on-coming friction element control force profile 87 through an open-loop based approach. An actual on-coming friction element control force 88 may differ from the commanded profile 87 due to control system variability. Even if actual control force 88 closely follows commanded profile 87, on-coming friction element engagement torque may still vary significantly from shift to shift due to the sensitivity of on-coming friction element engagement process to engagement conditions such as lubrication oil flow and friction surface temperature. Increasing on-coming friction element control force 88 or on-coming friction element engagement torque reduces net torque flow within the low-gear configuration. This contributes to output shaft torque 99 being reduced during torque phase 32, creating a so-called torque hole 85.
If the variability in on-coming friction element engagement torque significantly alters the shape and depth of torque hole 85, the vehicle occupant may experience inconsistent shift feel. A deep torque hole may be perceived as an unpleasant shift shock. During torque phase 32, off-going friction element control force is reduced as shown at 82 to induce an incipient slip 83. Controller 4 attempts to maintain off-going friction element slip at a target level through a closed-loop control based on off-going friction element speed 96 which may be directly measured or indirectly derived from speed measurements at pre-determined locations. A variability in off-going friction element control force 82 of off-going element slip torque may alter the shape and depth of torque hole 85, thus affecting shift feel. If controller 4 inadvertently allows a sudden increase in off-going friction element slip level, automatic transmission input speed or engine speed 90 may surge momentarily, causing the so-called engine speed flare or engine flare. The engine flare may be perceived by a vehicle occupant as an unpleasant sound.
Controller 4 initiates off-going friction element release process at a predetermined timing shown at 72 which may be based on a commanded value of on-coming friction element control force 93. Controller 4 lowers off-going friction element control force, following a pre-calibrated profile 94. If a release of off-going friction element D is initiated prematurely before on-coming friction element C develops enough torque, engine speed or input shaft speed may rise rapidly in an uncontrolled manner. If this engine speed flare 90 is detected, controller 4 increases off-going friction element control force to delay off-going friction element release process. Alternatively, to the pre-determined off-going friction element release timing, controller 4 may utilize speed signals to determine a final off-going friction element release timing. When a sign of significant gear tie-up, which may be manifested as a measurable drop in input shaft speed, is detected, off-going friction element D is released following a pre-calibrated force profile. Inertia phase 33 begins when off-going friction element torque capacity or control force drops to non-significant level 95. During inertia phase 33, off-going friction element slip speed rises 96 while on-coming friction element slip speed decreases 97 toward zero. The transmission input shaft speed drops as shown at 98 as the planetary gear configuration changes. During inertia phase 33, the output shaft torque 99 is primarily affected by on-coming friction element torque capacity, which is indicated by its control force 100. When on-coining friction element C becomes securely engaged at 101, the shift event completes.
In summary, a prior art methodology, which is based on an open-loop on-coming friction element control during a torque phase, cannot account for control system variability and dynamically-changing shift conditions during the torque phase, resulting in inconsistent shift feel or unpleasant shift shock. A pre-determined off-going friction element release timing with a pre-calibrated control force profile cannot ensure an optimal timing under dynamically changing shift conditions, resulting in inconsistent shift feel or unpleasant shift shock. The alternative approach to gauge off-going friction element release timing based on speed signals often results in a hunting behavior between gear tie-up and engine flare, leading to inconsistent shift feel. Furthermore, off-going friction element slip control is extremely difficult because of its high sensitivity to slip conditions. In addition, a large discontinuity exists between static and dynamic friction coefficients, introducing a large torque disturbance during an incipient slip control. A failure to achieve a seamless off-going friction element slip control during the torque phase leads to undesirable shift shock.
As can be seen from the above discussion the controllability of both off-going friction element and on-coming friction element is desirable during a torque phase in order to deliver a consistent and seamless shill quality. The prior art that depends on an open-loop approach, which may be based on speed measurements, does not have any solution to the problem of consistently controlling torque passing through either a multiple disc clutch or a band brake and therefore there is a need in the art for a transmission control system that minimizes shift shock during a gear ratio change that does not rely solely on traditional speed signal measurement and instead relies on measured or estimated friction element load level in either a multiple plate clutch or a band brake during a torque phase of gear-ratio changing.